A concern in the use of optical fiber amplifiers is to enhance the amplifier gain flatness across the amplifier nominal signal gain bandwidth which is critical for WDM optical communications, particularly for dense WDM (DWDM) communications. Also, in the case of single-channel analog (CATV) applications, the combination of source chirp and gain tilt results in signal distortion. Thus, gain slope should be minimized as well. Previously published results indicated that for erbium doped fiber amplifiers (EDFAs), gain saturation was essentially homogeneous. Hence, the varying of pump wavelength in the 980 nm band, for example, would only affect the overall gain level regardless of the signal wavelength and the noise figure of the fiber amplifier. Recently, however, in the article of Kevin W. Bennett et al. entitled, "980 nm Band Pump Wavelength Tuning of the Gain Spectrum of EDFAs", OAA '97, Optical Amplifiers and Their Applications, page 13 et al., paper PD4, which is incorporated herein by its reference, it was demonstrated that the changing of pump wavelength of a single pump source to a fiber amplifier, such as by varying pump drive current or by adjustment of pump laser temperature, can be sufficient to alter the gain spectrum of an EDFA and, in some cases to extend the gain flat dynamic range of an EDFA.
What is further desired is to provide independent adjustment of the relative power level over a plurality of amplifier pumps, operating at different wavelengths within an absorption band of the fiber amplifier pumped by the amplifier pumps, to change the weighted average or the effective combined wavelength of the combined pump wavelengths to modify the amplifier gain profile, in particular, flatten the gain profile or minimize its gain tilt.
Thus, it is an object of this invention to provide a fiber amplifier optimized for gain flatness.
It is another object of this invention to utilize co-propagating pump sources or a combination of co-propagating and counter-propagating pump sources in a manner to achieve improved signal gain response across the signal gain bandwidth of the amplifier.